This is a Phase III randomized, blinded study of nevirapine versus nevirapine placebo for the prevention of maternal-fetal transmission in pregnant HIV-infected women who present during the third month of pregnancy. The primary objective is to determine the effect of nevirapine on the incidence of mother-to-child transmission of HIV. The second objective is to evaluate the safety and tolerance of oral nevirapine in pregnant HIV-infected women and their infants. In addition, nevirapine pharmacokinetic parameters will be evaluated, as well as the relationship between such parameters and transmission of HIV. The relationship between maternal viral load at delivery and transmission will be evaluated in the nevirapine and placebo group. Approximately 900 of the required 1244 mother-infant pairs have been enrolled nationwide.